1. Field of the Invention
This invention relates to a cylinder air/fuel ratio estimation system of an internal combustion engine, more particularly to a system for estimating air/fuel ratios of each cylinder of a direct injection spark ignition multi-cylinder engine, in which gasoline fuel is directly injected into the combustion chamber of the engine.
2. Description of the Related Art
In an internal combustion engine having a plurality of cylinders, when only a single air/fuel ratio sensor is installed at or downstream of the confluence point of an exhaust manifold, since the sensor output merely indicates a mixed value of the air/fuel ratios exhausted by the whole cylinders, it becomes impossible to detect the air/fuel ratio in each cylinder accurately. In view of the above, the assignee proposes, in U.S. Pat. No. 5,524,598, estimating the air/fuel ratio in each cylinder based on a single air/fuel ratio sensor installed at or downstream of the confluence point of an exhaust manifold, based on a model established on the assumption that the sensor output indicates a weight-average value obtained by multiplying the past firing histories of the respective cylinders by a weight coefficient and an observer established to observe the internal state of the exhaust manifold expressed by the model.
Further, the time (or crank angles) at which the generated exhaust gas reaches or arrives the air/fuel ratio sensor varies with the engine operating conditions and some similar factors. Accordingly, the assignee proposes, in U.S. Pat. No. 5,600,056, sampling the sensor output successively and selecting one from among the sampled data by retrieving mapped data by engine operation parameters.
Furthermore, since the weight coefficient of the above-mentioned model varies with the engine operating conditions, the assignee proposes in U.S. Pat. No. 5,548,514, selecting and using one from among a plurality of observer gain matrices by similar engine operation parameters.
Aside from the above, a direct injection spark ignition engine has recently been proposed, as is disclosed in, for example, Japanese Patent Publication No. Hei 4 (1992)-37264. In the direct injection spark ignition engine, the engine operation is selected from one from three modes including a stoichiometric air/fuel ratio operation mode in which the desired air/fuel ratio is set to be the stoichiometric air/fuel ratio, a pre-mixture combustion operation mode (lean-burn operation mode) in which the desired air/fuel ratio is set to be an air/fuel ratio is leaner than the stoichiometric air/fuel ratio, and a stratified combustion operation mode (lean burn combustion operation mode) in which the desired air/fuel ratio is set to be a more leaner air/fuel ratio than that in the pre-mixture combustion operation mode, in response to the engine load.
In this kind of engine, even when the engine load remains unchanged, the aforesaid time (or crank angles) at which the exhaust gas reaches the air/fuel ratio sensor may vary with the respective operation modes, since the exhaust gas flow rate (or volume) varies with the operation modes. Further, the behavior of the air/fuel ratio sensor caused by the exhaust gas may be different for different operation modes.
An object of the present invention is therefore to provide a cylinder air/fuel estimation system of an internal combustion engine operable in a selected one of operation modes, which can select one from among sampled air/fuel ratio data correctly such that the air/fuel ratio at each cylinder can be accurately estimated for the selected operation mode.
This invention achieves this object by providing a system for estimating an air/fuel ratio of each cylinder of an internal combustion engine having a plurality of cylinders which are connected to an exhaust system having an exhaust manifold, including: an air/fuel ratio sensor installed at or downstream of a confluence point of the exhaust manifold and generating an output indicative of an air/fuel ratio exhausted from the cylinders; air/fuel ratio sensor output sampling module which A/D converts the output of the air/fuel ratio sensor and stores successively as sampled data; engine operating condition detecting module which detects operating conditions of the engine including at least an engine speed and an engine load; sampled data selecting module which selects one from among the sampled data based on at least the engine speed and the engine load in the detected operating conditions of the engine, and estimates the air/fuel ratio of each cylinder from the selected sampled data based on a model describing a behavior of the exhaust manifold and designed based on assumption that the output of the air/fuel ratio sensor comprises a weight-average obtained by multiplying past firing histories of the cylinders by a weight coefficient and an observer for observing an internal state of the model. The characteristic features of the system are that the system includes: operation mode selecting module which selects an operation mode of the engine from at least a first operation mode in which a desired air/fuel ratio is set to be a stoichiometric air/fuel ratio, a second operation mode in which the desired air/fuel ratio is set to be an air/fuel ratio leaner than the stoichiometric air/fuel ratio and a third operation mode in which the desired air/fuel ratio is set to be an air/fuel ratio leaner than that of the second operation mode; and the sampled data selecting module selects the one from among the sampled data based on at least the engine speed, the engine load and the selected operation mode, and estimates the air/fuel ratio of each cylinder from the selected sample, based on the model and the observer, for the selected operation mode of the engine.